The present invention relates to a dosing valve and to a dosing device for introducing a liquid medium into an exhaust gas stream of an internal combustion engine of a motor vehicle.
For internal combustion engines, compliance with pollutant emissions limits in the exhaust gas is legally required. Particularly in the case of a diesel vehicle, nitrogen oxide reduction is absolutely essential. One possibility for nitrogen oxide reduction is the known method of selective catalytic reduction (SCR), for example. In this method, a liquid reducing agent, e.g. an aqueous urea solution (“AdBlue”), is introduced into the exhaust gas stream in the exhaust pipe. With the hot exhaust gas, the aqueous urea solution is converted into gaseous ammonia, by means of which the toxic nitrogen oxide is reduced to form harmless water and nitrogen.
Pressure-wave-controlled dosing systems are known. In these systems, the injection valve consists exclusively of mechanical components. A valve needle is held shut by means of a spring holder and opens automatically above a certain hydraulic pressure. Injection is thus controlled by means of a pressure wave, which is produced with the aid of an external pump. One disadvantage is the low accuracy of metering since the injection quantity is heavily dependent on the shape of the pressure wave, which can easily be affected by external influences.
Patent Application DE 10 2011 078 852 A1, which is a post-publication, discloses another embodiment of a dosing valve, in which a pump and a nozzle are integrated in one unit. This integrated dosing module (IDM) does not have a return. The pump is designed in such a way that it also assumes the function of metering an accurate quantity of the liquid medium. This is achieved by virtue of the fact that the pump delivers in a purely volumetric way, with the result that a defined quantity of the medium is delivered with each delivery stroke. Very good metering accuracy is thereby achieved. One disadvantage in the system is the complex production thereof. Currently known embodiments contain a control edge, which must be set precisely, and a plurality of guides, which must be manufactured with an accurate fit relative to one another.
DE 10 2008 001 789 A1 shows a dosing device in which a dosing valve is controlled by an electromagnet. In this case, a flow rate of the aqueous urea solution when the valve is completely open is determined in accordance with a pressure of the aqueous urea solution. For this purpose, use is made of a characteristic which represents a relationship of the steady state flow rate against the pressure. This characteristic shows a relationship between the activation duration of the electromagnet and the injection quantity. The dosing of the aqueous urea solution is controlled by suitable activation of the valve.